This invention relates to a dielectric liquid impregnant for electrical devices and more particularly to an electrical capacitor utilizing a unique combination of gas absorbing impregnant materials which provide an improved dielectric liquid impregnant theretofore.
Liquid impregnants for electrical capacitors should have a high dielectric constant, maintain a low dissipation factor, and be compatible with the other materials utilized in capacitor structure. At the same time, the impregnant must withstand elevated and fluctuating temperature, pressure, and voltage stress conditions with excellent electrical characteristics for a long operative life of the capacitor.
The polychlorinated biphenyls as capacitor impregnants meet these requirements and were eminently satisfactory for several decades. The polychlorinated biphenyls, a term which is inclusive of chlorinated diphenyl as used herein, are broadly referred to as PCBs. The polychlorinated biphenyls have recently been associated with ecological problems, restrictive use limitations, and rising costs. These problems of chlorinated diphenyls has spurred the search for a suitable replacement capacitor impregnant which would have some advantageous impregnant characteristics comparable to those of the chlorinated diphenyls, and still provide outstanding electrical and compatibility performance with the two most important present day capacitor solid dielectrics, paper and polypropylene. A primary candidate for a replacement impregnant is an ester base fluid. However, in the utilization of paper and polypropylene dielectric materials with ester based impregnants, it was found that, in the capacitor environment, principally hydrogen gas was evolved from the ester or the environment and by contributing to ionization and corona discharge shortened the effective life of the capacitor. Gas evolution, under certain capacitor conditions of elevated temperatures and high electrical stresses appears to be a problem with other candidate impregnants although the esters and some other candidates are ordinarily gas absorbing materials. Accordingly, it is an object of this invention to provide an improved hydrogen gas stable impregnant for electrical devices.